As is well known, a simulcast system employs a plurality of base station RF transmitters to transmit an identical message to a mobile receiver. The transmitter geometry is typically a rectangular zone configuration, although triangular, hexagonal, and polygon "tile" configurations are also well known.
A receiver that is positioned within an overlap region between two transmitters simultaneously receives two signals that differ in arrival time and phase due to differences between when the signals are transmitted, multipath delays experienced by the transmitted signals, and other factors. However, in order to obtain an optimum received signal it is desirable that these identical simultaneous transmissions arrive at the receiver aligned in both time and phase.
For example, if, in a digital simulcast system operating at 9.6 Kbps, the transmissions are misaligned by more than 36 microseconds, the resulting mutual interference may cause erroneous reception of the data. As a result, present-day simulcast systems require the transmitted signals to be aligned in amplitude and phase so that the difference is less than 0.5% across the audio band, on a per channel basis. In order to maintain alignment between the plurality of transmitters, a time consuming calibration process must be performed at the individual base stations. One calibration method used to align the multiple transmissions requires 30 different multi-turn trimpot adjustments per channel/base station site. This calibration method requires precise adjustment of the trimpots and is extremely time consuming. Furthermore, periodic calibration must be performed in order to account for system drifts.
Other techniques for attempting to reduce the effect of misalignment between simulcast transmitters are known in the prior art. For example, in an article entitled "Multitransmitter Digital Signal Transmission by Using Offset Frequency Strategy in a Land-Mobile Telephone System" IEEE Transactions on Vehicular Technology, Vol. VT-27, No.4, November 1978, T. Hattori and K. Hirade describe a high capacity land-mobile telephone system in which voice channels are provided separately from control channels. A multiple control zone system that employs an offset carrier frequency technique is shown to be a preferred method for minimizing signal degradation . In an article entitled "Delay Spectrum Method Using Cross-Spectrum For Multitransmitter Simulcasting", Electronic Letters, May 11, 1989, Vol. 26, No. 10, J. Oka, T. Hattori and S. Ogose propose a solution based upon a delay difference detection method using cross-spectrum phase. In this scheme the multitransmitter simulcasting system employs a detector to detect delay differences between transmitted signals and a means for appropriately adjusting the phase of the input signals to transmitter transmission lines to minimize the detected delay differences. Delay differences between two signals are detected as the phase angle of the cross-spectrum of the two signals. The authors state that estimation of the delay is improved by employing a pseudorandom noise (PN) signal as a test signal in conjunction with a Minimum Square Error Estimation technique. The Hattori et al. and Oka et al. articles are hereby incorporated by reference.
These techniques, however, do not provide for improving signal quality automatically within the mobile receiver itself.
It is thus an object of the invention to provide method and apparatus for achieving automatic simulcast alignment in a digital simulcast system within the mobile receiver itself.
It is a further object of the invention to provide method and apparatus for achieving automatic simulcast alignment in a digital simulcast system by considering the multiplicity of received signals as a multipath delay problem and employing an adaptive recursive equalizer to correct the misalignment in time and to correct for phase and amplitude differences.
It is a further object of the invention to provide method and apparatus for achieving automatic simulcast alignment in a digital simulcast system by employing an adaptive equalizer to correct the misalignment in time and to correct for phase and amplitude differences while also beneficially providing diversity gain by combining energy from each received signal.